Apparatus for atmospheric pressure reactive atom plasma processing for surface modification

ABSTRACT

Reactive atom plasma processing can be used to shape, polish, planarize and clean the surfaces of difficult materials with minimal subsurface damage. The apparatus and methods use a plasma torch, such as a conventional ICP torch. The workpiece and plasma torch are moved with respect to each other, whether by translating and/or rotating the workpiece, the plasma, or both. The plasma discharge from the torch can be used to shape, planarize, polish, and/or clean the surface of the workpiece, as well as to thin the workpiece. The processing may cause minimal or no damage to the workpiece underneath the surface, and may involve removing material from the surface of the workpiece.

CLAIM OF PRIORITY

This application is a divisional application of the followingapplication:

-   U.S. patent application Ser. No. 10/002,483, entitled APPARATUS AND    METHOD FOR ATMOSPHERIC PRESSURE REACTIVE ATOM PLASMA PROCESSING FOR    SURFACE MODIFICATION, inventor Jeffrey W. Carr, filed Nov. 1, 2001    (Attorney Docket No. CARR-01000US2), which claims priority from the    following application that is hereby incorporated by reference in    its entirety:

U.S. Provisional patent application No. 60/265,332, entitled APPARATUSAND METHOD FOR ATMOSPHERIC PRESSURE REACTIVE ATOM PLASMA PROCESSING FORSHAPING OF DAMAGE FREE SURFACES, filed Jan. 30, 2001 (Attorney DocketNo. CARR-01000US0).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending applicationwhich is hereby incorporated by reference in its entirety:

J U.S. patent application Ser. No. 10/002,035, entitled APPARATUS ANDMETHOD FOR ATMOSPHERIC PRESSURE REACTIVE ATOM PLASMA PROCESSING FORSHAPING OF DAMAGE FREE SURFACES, inventor Jeffrey W. Carr, filed Nov. 1,2001 (Attorney Docket No. CARR-01000US1)

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California.

FIELD OF INVENTION

The field of the invention relates to shaping surfaces using a gasplasma.

BACKGROUND

Modern materials present a number of formidable challenges to thefabricators of a wide range of optical, semiconductor, and electroniccomponents, many of which require precision shaping, smoothing, andpolishing. Current methods, such as conventional grinding and polishing,have a number of disadvantages. Physical contact methods, such asgrinding, abrasive polishing, diamond turning and ion milling, involvephysical force at the microscopic scale and can create damage in thesubsurface of the material being treated. Physical contact methods alsohave a trade-off between speed and quality. Smooth surfaces can requirethe use of very slow material removal rates, while hard materials suchas silicon carbide can be extremely difficult to polish. Soft ordelicate structures can also be difficult to polish, as the physicalforce involved can crack or bend the structures. Some materials such asglass can also end up with a surface layer of redeposited material,which can affect the properties and behavior of the manufacturedcomponent.

Damage-free Laser Optics

In one example of such a manufacturing challenge, optics produced withcurrent or prior art polishing methods cannot withstand the highintensity of light produced by high-powered laser systems. One of theengineering challenges in such an advanced system is the need for alarge number of defect-free optics to be produced within an acceptableperiod of time and at an acceptable cost. Subsurface defects in such anoptic can cause cracks to form on the rear surface of lenses exposed tohigh ultraviolet laser light levels. These cracks can grow until a largefraction of the light is obscured or until the lens fractures. Some ofthese lenses also serve as a vacuum barrier, making catastrophic failurea serious safety concern.

Conventional abrasives-based polishing can be used for many materials.This polishing process is both chemical and mechanical, involvingsurface and solution chemistry as well as mechanical abrasion.Mechanical abrasion rapidly removes material, but can producesub-surface damage and cause the damage to propagate deeper into theworkpiece. The chemical portion dissolves and redeposits glass, forminga relatively smooth surface. The chemical kinetics of redeposition favorthe formation of smooth surfaces, as high spots are mechanically abradedaway while holes are filled through redeposition.

This process of redeposition can lead to problems in some applications.Analysis of the redeposition layer reveals a tremendous number ofcontaminants, mostly from the abrasive but also from previous polishingsteps. This redeposition layer can affect the adhesion and physicalproperties of optical coatings. Below this redeposition zone can be anunderlying zone of damaged glass, up to tens of microns thick or more.When high fluxes of light pass through this zone, damage sites cannucleate and grow, eventually leading to failure of the entire optic.The quality of the polish, and the underlying redeposition layer andsubsurface damage, ultimately control how much light can be transmittedthrough the optics.

In order to produce optics capable of routinely withstanding laserintensities as high as 12 J/cm², a process is required to remove the20-30 microns of damaged material. Conventional polishing can be used toremove this damage layer, but it must be done very slowly—on the orderof about 0.1 μm per hour. Polishing for this length of time alsonecessitates periodic checks of the shape of the part using precisionmetrology.

Wet Etching

Another approach to removing the damage layer in an optic is a wetchemical etch. In such a process, only a limited amount of material canbe removed before the surface becomes excessively pitted, with aresulting increase in the amount of light scattered by the optic. Opticsprocessed by wet etch have been tested, with the disappointing resultthat the damage threshold was unaffected.

Ion Milling

Another approach utilizes ion milling after conventional polishing. Ionmilling is a well-established technique for removing small amounts ofmaterial from a surface using a kinetic beam of ions. Some advantages ofion milling include: no surface contact, no weight on the optic, no edgeeffects, and correction of long spatial wavelength errors.

There are numerous disadvantages to ion milling, however, including highsurface temperatures, an increase in surface roughness, and the need forvacuum. The temperature is dependant on beam current, so that anincrease in etch rate produces an increase in temperature oftensurpasses several hundred ° C. Nearly all heat must be removed throughthe chuck, usually requiring a good thermal connection between theworkpiece and the holder. This is difficult when working on transmissionoptics because they must be held by the edges so as not to damage thepolished surface. Further, ion beams cannot smooth surfaces. For smallamounts of material removal, roughness can be held constant. Largeamounts of material removal cause an unfortunate increase in roughness.

Reduced Pressure Plasma Methods

Another approach involves plasma etching at reduced temperature, whichis used extensively in the semiconductor industry for processing of awide variety of materials including semiconductors, metals and glasses.Reactive ions are believed to be responsible for the majority ofmaterial removal, leading the technique to be known as reactive ion etch(RIE). Considerable effort has been put into developing plasmas withuniform etch rates over the entire discharge, making RIE unsuitable forthe production of figured precision components. The greatest practicaldrawback to RIE for precision finishing of optical components is theneed for vacuum and a low material removal rate. Translating either thesource or workpiece with precision on a complicated path inside a vacuumchamber is challenging, especially in the case of large optics. In-situmetrology would also be awkward.

A modified RIE for polishing at reduced pressure has been built using acapacitively coupled discharge. Named “Plasma Assisted ChemicalMachining” (PACE), the system has been successful in shaping andpolishing fused silica. While the parts polished by PACE have shown noevidence of subsurface damage or surface contamination, it has beenfound that greater sub-surface damage present before etching resulted inan increased roughness after etching.

A major limitation of this capacitively-coupled discharge approach isthe requirement that the workpiece be either conductive or less than 10mm thick. In addition, etch rates are dependant on part thickness,decreasing by a factor of ten when thickness changed from 2 to 10 mm.Above 10 mm the rates are too low to be of much use. If metrology isneeded in an iterative procedure, the chamber must be vented and pumpeddown for the next etch step. The convergence rate for PACE is alsotypically very low, resulting in a long, expensive multi-step process.PACE technology was recently improved by the substitution of a microwaveplasma source for the capacitively coupled system, but the rates arestill too slow for optics manufacturing.

Atmospheric Pressure Plasma Methods

In yet another approach, a direct current (DC) plasma can be used atatmospheric pressure to thin wafers. Originally called a “Plasma Jet”and also referred to as Atmospheric Downstream Plasma (ADP), such asystem uses argon as the plasma gas, with a trace amount of fluorine orchlorine for reactive atom production. The main intent of the device isto do backside thinning of processed silicon wafers for smart card andother consumer applications. With the ADP tool, wafers are thinned in abatch mode by placing them on a platten and using planetary type motionto move the sub-aperture plasma in a pseudo-random fashion across thesurface.

Unfortunately, atmospheric DC plasma jets such as ADP are not wellsuited for the precise shaping and smoothing of surfaces or for materialdeposition. Because the reactive gas is mixed with the bulk gas prior toexcitation, the reactive species in the plasma are widely distributedacross the discharge. This substantially increases the footprint and theminimum feature size that can be etched into a surface. Furthermore, theelectrodes that are used to establish the arc are eroded by thereactants. This adds particulates to the gas stream, as well as causingfluctuations in plasma conditions, and accounting for the reduceduniformity compared to RIE systems. Detrimental electrode reactions alsopreclude the use of oxygen and many other plasma gases.

Another plasma process, known as Chemical Vapor Machining (CVM), is aradio frequency (RF) plasma process that has been used to slice silicon.This plasma is generated around a wire or blade electrode immersed in anoble gas atmosphere containing a trace of reactive components. Like thePACE process it closely resembles, material removal through CVM isentirely chemical in nature. The damage for CVM and wet chemical etchingare similar, close to the intrinsic damage typically found in siliconused in the semiconductor industry.

Several performance characteristics limit the applications of CVM.First, the non-rotationally symmetric nature of the footprint makes theprocess difficult to model and control. Process rates are limited by therate at which the plasma converts the reactive precursor gas intoradical atoms. The device is difficult to scale up, limiting the maximumremoval rate and the practical limit for fine-scale material removal.While no vacuum is required for CVM, the workpiece must be enclosed in avessel to contain the plasma atmosphere.

Another type of plasma jet has been developed to etch and depositmaterial on surfaces as well as to clean surfaces, known as an “ApJet.”This system consists of two concentric electrodes that generate a DCplasma which exits through a nozzle. The discharge is at a lowtemperature, making the process suitable for cleaningtemperature-sensitive materials. The ApJet is not suitable for preciselyshaping and polishing surfaces, as etch rates are low and the electrodesand nozzle erode and deposit material onto the surface. This makesprecision control difficult. Furthermore, the ApJet cannot smooth roughsurfaces.

BRIEF SUMMARY

Systems and methods in accordance with the present invention overcomedeficiencies and obstacles in the prior art to produce ahighly-controllable, precise, atmospheric, non-contact material removalprocess. These systems and methods also provide improved processes forshaping geometric surfaces and rapidly shaping hard-to-machinematerials, as well as rapidly thinning finished silicon devices withhigh smoothness and minimal thickness variation.

One method for shaping a surface of a workpiece involves placing theworkpiece in a plasma processing chamber that includes a plasma torch,such as an ICP torch. The workpiece and plasma torch are moved withrespect to each other, whether by translating and/or rotating theworkpiece, the plasma, or both. Reactive atom plasma processing is usedto shape the surface of the workpiece with the discharge from the plasmatorch. Reactive atom plasma processing can also be used for purposessuch as to planarize, polish, clean, or thin the workpiece. Theprocessing may cause minimal or no damage to the workpiece underneaththe surface, and may involve removing material from the surface of theworkpiece.

Also included in the present invention are tools and systems foraccomplishing these and other methods. Such a system for shaping thesurface of a workpiece can involve a plasma torch configured to shapethe surface of a workpiece using a reactive plasma process. A translatorcan be used to translate the workpiece, the torch, or both, such thatthe desired shape, planarization, polishing, or cleaning is achieved.The torch can be contained in a plasma processing or other appropriatechamber.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram of a system in accordance with one embodiment of thepresent invention.

FIG. 2 is a diagram of the ICP torch of FIG. 1.

FIG. 3 is a diagram showing relative concentrations of reactive atomsand reactive ions in a plasma discharge that can be used in accordancewith one embodiment of the present invention.

FIG. 4 is a graph of a footprint of a tool that may be used inaccordance with one embodiment of the present invention.

FIG. 5 is a flowchart showing a process in accordance with oneembodiment of the present invention.

FIG. 6 is a flowchart showing another process in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION

Systems and methods in accordance with the present invention haveadvantages over prior art systems, such as PACE and chemical vapormachining, in that the number of potential products increases to includedevices fabricated from heat sensitive components and heterogeneousmaterials that are typically difficult to polish by chemical means.Polishing and planarization are now be possible with little heat gainand minimal material removal.

FIG. 1 shows one embodiment of a reactive atom plasma (RAP) system thatcan be used in accordance with the present invention. FIG. 1 shows anICP torch in a plasma box 106. The torch consists of an inner tube 134,an outer tube 138, and an intermediate tube 136. The inner tube 134 hasa gas inlet 100 for receiving a reactive precursor gas from the massflow controller 118. The intermediate tube 136 has a gas inlet 102 forreceiving an auxiliary gas from the flow controller 118. The outer tube138 has a gas inlet 104 for receiving a plasma gas from the mass flowcontroller 118. The mass flow controller 118 receives the necessarygasses from a number of gas supplies 120, 122, 124, 126, and controlsthe amount and rate of gasses passed to the respective tube of the ICPtorch. The ICP torch generates a plasma discharge 108, which can be usedto, for example, shape or polish a workpiece 110 located on a chuck 112in the workpiece box 114. In this embodiment, the plasma box 106 andworkpiece box 114 are separate, allowing the plasma discharge 108 and/ortorch to pass at least partially between the plasma 106 box and theworkpiece box 114. The workpiece box 114 has an exhaust 132 for carryingaway any process gases or products resulting from, for example, theinteraction of the plasma discharge 108 and the workpiece 110. In otherembodiments, there may not be separate boxes for the plasma torch andthe workpiece.

The chuck 112 in this embodiment is in communication with a translationstage 116, which is adapted to translate and/or rotate a workpiece 110on the chuck 112 with respect to the plasma discharge 108. Thetranslation stage 116 is in communication with a computer control system130, such as may be programmed to provide the necessary information orcontrol to the translation stage 116 to allow the workpiece 110 to bemoved along a proper path to achieve a desired shaping or polishing ofthe workpiece. The computer control system 130 is in communication withan RF power supply 128, which supplies power to the ICP torch. Thecomputer control system 130 also provides the necessary information tothe mass flow controller 118.

The torch itself can be seen in greater detail in FIG. 2. An inductioncoil 140 surrounds the outer tube 138 of the torch near the plasmadischarge 144. Current from the RF power supply flows through the coil140 around the end of the torch. This energy is coupled into the plasma.Also shown are the excitation zones 142, into which the reactiveprecursor is injected, and the plasma envelop 146, which can be forexample a sheath of argon gas.

One method for using such a system is shown in FIG. 5. In this method, aworkpiece is placed in a plasma processing chamber that includes aplasma torch 500. At least one of the workpiece and the plasma torch istranslated and/or rotated, such as by translating the workpiece withrespect to the torch 502. Reactive atom plasma processing is then usedto shape the surface of the workpiece with the discharge from the plasmatorch 504.

In another method, shown in FIG. 6, the workpiece is again placed in aplasma processing chamber including a plasma torch 600. A controlledflow of precursor is placed in a central channel of the plasma torch602. A plasma gas is introduced through an outer tube 604, and anauxiliary gas is introduced through an intermediate tube of the plasmatorch 606. The gases can be introduced relatively simultaneously. Energyis coupled to the plasma discharge in an annular region of the plasmatorch 608. At least one of the workpiece and the plasma torch istranslated and/or rotated, such as by translating the workpiece withrespect to the torch 610. Reactive atom plasma processing is then usedto shape the surface of the workpiece with the discharge from the plasmatorch 612.

Chemistry

A reactive atom plasma process in accordance with the present inventionis based, at least in part, on the reactive chemistry of atomic radicalsformed by the interaction of a non-reactive precursor chemical with aplasma. In one such process, the atomic radicals formed by thedecomposition of a non-reactive precursor interact with material on thesurface of the part being shaped. The surface material is transformed toa gaseous reaction product and leaves the surface. A variety ofmaterials can be processed using different chemical precursors anddifferent plasma compositions. The products of the surface reaction inthis process must be a gas under the conditions of the plasma exposurefor etching to take place. If not, a surface reaction residue will buildup on the surface which will impede further etching.

In one process in accordance with the present invention, the chemistryis specific to fluorine and materials that react with fluorine to formgaseous products. Following are three specific examples where weightloss was measured. The materials processed include:

-   -   Silicon dioxide (fused quartz) where the balanced reaction of        concern is        SiO₂+CF₄->SiF₄+CO₂    -   Silicon carbide works with or without the addition of O₂. The        use of O₂ can greatly speed the operation. One such balanced        equation is given by:        SiC+CF₄₊₂O₂->SiF₄₊2CO₂    -   Silicon works with or without addition of oxygen to the plasma.        Oxygen can also be supplied by the ambient air. A balanced        equation that can be used with the process is given by:        Si+CF₄+O₂->SiF₄+CO₂        The reaction may also work with CF₄ supplied by the device and        ambient oxygen in the tool enclosure.

Other fluorocarbons and molecules containing fluorine can work as well.SF₆ has been used as the chemical precursor to successfully etch silicaglass. The equation can be the same as for CF₄, such as may be given by:3SiO₂+2SF₆->3SiF₄+2SO₂+O₂

-   -   or        3SiO₂+2SF₆->3SiF₄+2SO₃        In addition to SF₆, a large number of fluorine-containing        chemicals may be suitable for use as reactive precursors. For        example, chemicals of the type C_(n)F_(2n+2), such as C₂F₆,        C₃F₈, and C₄F10 can be used. Fluorine chemicals with other        cations may also be suitable, as well as F₂. For work on        materials that do not contain silicon, such as, but not limited        to, oxides, metals, carbides, and organic materials, a different        reactive atomic species may be appropriate, such as chlorine or        bromine. Compounds containing these elements may also be        suitable as reactive precursors. An example of such a suitable        class of chemicals would be the class of halocarbons. Mixtures        of more than one reactive precursor can also be used.

In the above examples, the reactive precursor chemical can be introducedas a gas. Such a reactive precursor could also be introduced to theplasma in either liquid or solid form. Liquids can be aspirated into theplasma and fine powders can be nebulized by mixing with a gas beforeintroduction to the plasma. In fact, an aqueous solution of HF can beparticularly effective because it supplies both fluorine for etching andoxygen for carbon removal, if needed. The equations for such a processmay be given by:SiO₂+4HF->SiF₄+2H₂O

-   -   or        SiC+4HF+2O₂->SiF₄+CO₂+2H₂O        Such a process has several advantages over the RIE process. RIE        requires a vacuum, whereas RAP processing can be used at        atmospheric pressure. RAP has much higher material removal rates        and can be used as a sub-aperture tool to precisely shape        surfaces, whereas RIE is best suited to remove small quantities        of material across an entire surface. Finally, RIE cannot smooth        rough surfaces whereas RAP processing rapidly polishes and        etches surfaces.        ICP Plasma Torch

An inductively-coupled plasma (ICP) is an excellent source of reactiveatoms useful for shaping damage free surfaces. An ICP discharge haspreviously been used to produce crystalline films of a number of oxides,such as MgO, ZrO₂, NiO, SnO₂, TiO₂, ZnCr₂O₄, Cr₂O₃, CoCr₂O₄, NiCr₂O₄,and several rare earth oxides. Superconducting thin films ofBi—Pb—Sr—Ca—Cu—O have also been fabricated with ICP plasma spraymethods.

The high electrical conductivity of partially ionized gases (forexample, 120 ohm/cm-1 at 15,000° K. for argon) may contribute to theease of inductively coupled plasma formation at high pressures. ICPsystems do not require electrodes. A number of gases can be used as thehost plasma, though argon may be the principle component. A typicaldischarge can be characterized by a high current (such as 100 to 1000amps) and a relatively low voltage (such as 10 to 100 volts). Theflowing plasma is not in complete thermodynamic equilibrium, but ion andexcited state atom populations can be within 10% of equilibrium values.Electron densities can be high, typically above 10¹⁵ cm⁻3, whichsuggests electron temperatures above 15,000K. A peak temperature of10,000K can be calculated from the ratio of emission intensities for aset of argon lines (again assuming equilibrium) and gas kinetictemperatures have been estimated to be roughly 6,000K. These hightemperatures make the ICP an efficient source for the generation ofreactive atoms.

The current from a 27.12 MHz RF generator flows through a three turncopper load coil around the top of the torch, such as the one shown inFIG. 2. The energy is coupled into the plasma through an annular “skinregion” that is located on the outer edge of the plasma nearest the loadcoil. The plasma can be supported in a quartz tube by the plasma gas,which can be introduced tangentially to form a stabilizing vortex. The“skin region” is thinnest along the central axis and the droplets or gaseasily penetrate the discharge. As the droplets travel through theplasma they becomes progressively desolvated, atomized, excited, andionized. The relative distribution of ions and atoms in the discharge isrepresented in FIG. 3. Spatial profiles at five places in the plasmaindicate that the excited ion population decays faster than that of theneutral atoms, most likely a result of ion-electron recombination. Themaximum atomic emission from the material injected into the plasmaoccurs several millimeters above the load coil near the visible tip ofthe discharge (zone 3 and 4). Radiative decay in this region is used tospectroscopically determine the composition of the injected material.

A standard, commercially-available three tube torch can be used, such asone having three concentric tubes as discussed above. The outer tube canhandle the bulk of the plasma gas, while the inner tube can be used toinject the reactive precursor. Energy can be coupled into the dischargein an annular region inside the torch. As a result of this coupling zoneand the ensuing temperature gradient, a simple way to introduce thereactive gas, or a material to be deposited, is through the center. Thereactive gas can also be mixed with the plasma gas, although the quartztube can erode under this configuration.

Injecting the reactive precursor into the center of the excitation zonehas several important advantages over other techniques. Some atmosphericplasma jet systems, such as ADP, mix the precursor gas in with theplasma gas, creating a uniform plume of reactive species. This exposesthe electrodes or plasma tubes to the reactive species, leading toerosion and contamination of the plasma. In some configurations of PACE,the reactive precursor is introduced around the edge of the excitationzone, which also leads to direct exposure of the electrodes and plasmacontamination. In contrast, the reactive species in the RAP system areenveloped by a sheath of argon, which not only reduces the plasma torcherosion but also reduces interactions between the reactive species andthe atmosphere.

The second of the three tubes, optional in some embodiments, can be usedto introduce an auxiliary gas, such as at a rate of about 1 L/min. Theauxiliary gas can have at least two functions. First, the gas can keepthe hot plasma away from the inner tube, since even brief contact mayseal the inner tube shut. Second, the gas can be used to adjust theposition of the discharge in space.

The inner diameter of the outer tube can be used to control the size ofthe discharge. On a standard torch, this can be on the order of about 18mm. In an attempt to shrink such a system, torches of a two tube designcan be constructed, which can have an inner diameter of, for example,about 6 mm, although larger or smaller inner diameters may beappropriate.

The outer tube gas, such as a plasma gas, can be introduced tangentiallyand can stabilize the discharge. The tangential introduction can also bemaintained with no auxiliary tube. A de-mountable system can be used,where the tubes are individually held and separately replaced. Anadvantage to such a system is that the length of the outer tube can belengthened, allowing the plasma to cool down while preventing reactiveradical atoms from reacting with air.

A small torch erosion problem may exist due to a minor portion of theprecursor not entering the central zone but instead going around theoutside of the plasma. An increase in skin depth (i.e. a thicker energycoupling zone) can constrict the central channel, possibly restrictingthe precursor flow and allowing some to escape to the periphery. One ofthe advantages of systems in accordance with the present invention isthat there is little to no electrode or nozzle erosion.

Housing

As shown in FIG. 1, there are several basic blocks to a system inaccordance with the present invention. A plasma box can be used to housethe ICP torch. The plasma box can be used, for example, to shield anoperator from radio frequency energy generated during a process, and/orfrom UV light produced by a plasma. The plasma box can be kept under aslight negative pressure, such as by hooking it up to a chemical hoodexhaust system. The entire enclosure can be constructed, for example,from a single sheet of copper that has been folded, rather thanconnected from individual plates.

One of the characteristics of RF is that it travels along a surface of ametal rather than through a metal. RF tends to find and leak out ofseams and around door frames. Since it may not be possible to completelyavoid edges, the edges of the box can be filled with, for example,silver solder and ground with a radius on them, so that there are nosharp points or edges. Pieces that move, such as doors, can be boltedtight, such as through the use of fasteners.

Holes and windows can be formed or cut into the box, such as to allowfor air to enter the plasma and sample box, as well as to allow accessfor servicing, and to provide a place for visual inspection of thesystem while operating. Since RF cannot escape from holes much smallerthan the wavelength of the radiation (for 13.56 MHz the wavelength invacuum is about 23 meters), a 100 mm square window can have very littleleakage. The windows can use welders glass, for example, and the serviceholes can be covered with copper tape or other UV-filtering material.

An aluminum sample box can be used to contain the workpiece andtranslation stages. Aluminum plates can be bolted together to form sucha box. It may be unnecessary to use copper, as there may be no need toshield from RF. The sample box can be connected directly to an adjoiningtorch box, such as through a circular hole. There can also be a windowto allow an operator of the system to watch the part during the process,as well as ventilation openings if necessary. A main exhaust system canbe connected to the top of the chamber, although other designs may havethe exhaust hose or the stage in a different location, such as mayminimize turbulence around the part. There can also be a gauge tomeasure the pressure differential between the room and the inside of thechamber.

The main components inside a sample chamber in accordance with thepresent invention, with the exception of the sample, are the translationstages and the chuck. The chuck can be a relatively simple vacuumsystem, which can be mounted to the rotary stage and connected to apump, such as a carbon vane pump, through a rotary or other appropriateconnection. The chuck can be smaller than, or equal in size to, the sizeof the part. If the chuck protrudes past the part, a small amount ofchuck material may deposit on the edge or surface.

Gas Flow Control

Devices such as rotometers and mass flow controllers can be used tometer gas flow. A system can, for example, use mass flow controllerswith piezoelectric transducers to monitor gas flow on all lines exceptthe auxiliary. A power source and control panel can be rack mounted.This can be a commercial unit useful for low pressure capacitivelycoupled discharges. The rack can also contain the stage controller andthe electronics for the mass flow controllers.

The introduction of reactive gas into the plasma can be controlled by amass flow controller over a range, for example, of 2000 ml of CF₄ perminute to 0.05 ml per minute, with an accuracy that may be in the rangeof +/−2.0%. With such a system, it may be possible to go from, forexample, 40 L/min of CF₄ (by using CF₄ in the main body of the plasma)to 0.01 ml/min (using dilution).

There can be several mass flow controllers controlling gas introduction.Having several controllers in series and/or parallel with flow rangessuch as from 10 L/min to 0.1 L/min can provide a great deal offlexibility, and allows for complex chemistries of reactive precursorgases. In one example, 1 ml/min of CF₄ is introduced into the centralchannel using such a system.

The main gas flow, such as may contain a plasma gas, can serve to supplythe discharge with a flowing stream of, for example, argon. The flowrate can be changed over a fairly wide range, such as from zero to about40 L/min. If the flow is too fast, the plasma may “blow out.” A largeflow rate can result in a dilution of both the reactive gas and of theenergy put into the system.

RF Power Supply and Control

A wide range of power conditions can be used when operating a system inaccordance with the present invention. Standard RF units operate at13.56 MHz, 27.12 MHz, or 40.68 MHz. The frequencies are presently set bythe FCC, and may not effect the performance of atomization but mayaffect the skin depth of the plasma. While a standard RF unit can have amaximum power of 5 to 10 kW, many applications may never require powerabove 2.5 kW.

At certain reactive gas flow rates, the additional power may do nothingbut deposit more heat on the part. Surface heating on the part can beimportant to reaction rates and reaction efficiency. Generally, therates increase with temperature. It may be undesirable to greatlyincrease the temperature of the part, as reaction products can beproduced that condense on cooler areas of the part and on the housing ofthe device. Too much heat can also cause thermal stress in the part, aswell as a change in shape due to thermal expansion. The additionalenergy at the high power settings can also serve to reduce the number ofactive species, such as by converting the reactive atoms to ions andreduce their reactivity.

In one system in accordance with the present invention, the process mustproduce a volatile reaction product to be successful. The plasmatemperature can be between 5,000 and 15,000° C. As the plasma can be anon-equilibrium system, different techniques for estimating temperaturecan yield different results. The lower value, 5,000° C., is the gaskinetic temperature and may bear the largest responsibility for heatingthe part.

The entire system can be mounted on an optical table, or any otherappropriate mounting surface or structure. Since the removal tool is agaseous flow of reactive atoms, it may not be very vibration sensitive.To eliminate any environmental contribution, a clean room or otherappropriate enclosure can be built around the sample chamber and torchbox.

Dynamic Range

One advantage of a system in accordance with the present invention isthe dynamic range of material removal. At a low setting, the reactivegas can be delivered in such minute quantities that single atomic layersare removed, such as over a period of seconds or even minutes. At highersettings, the process can remove at least grams of material per minute.While they might not be practical for material removal, very low etchrates can be important for modifying the surface of materials treatedwith the plasma.

By using a range of mass flow controllers and using precursor gas in100%, 10% and 1% mixtures with argon, a dynamic range of five orders ofmagnitude in etch rate is available in one embodiment, althoughadditional orders of magnitude in etch rate are possible using differentranges and mixtures. At a high end, such as may be achieved by confininga precursor to the central channel, it is possible to introduce 1000ml/min of 100% CF₄. On a low end, a 1% mix of CF₄ in argon can bedelivered to a central channel with a flow rate of 1 ml/min. Etch ratescan be reduced by two more orders of magnitude such as by using a flowcontroller that operates, for example, from 0 to 10 ml/min and/or by afurther 10× or other appropriate dilution of the gas.

Precision Shaping

Using conditions such as those described above, it is possible to get astable, predictable, reproducible distribution of reactive species thatis roughly Gaussian in nature, although other distributions are possibleand may be appropriate for certain applications. For many applications,it may only be desirable that the distribution be radially symmetric.For example, a 18 mm inner diameter torch may have a spread of about 30mm. FIG. 4 is a probe trace of a pit produced by a 1.5 kW plasma with areactive gas flow rate of 50 mls/minute over a 5 minute period. Thedistance from the load coils (energy induction zone) to the part surfacewas 25 mm. As the exposure time is increased or decreased, such a holecan get deeper or shallower, but its width may not vary greatly.Therefore, the tool shape produced by the plasma system can be extremelyshallow and broad, which can relax the requirements for precision X-Ypositioning of the tool or the part.

An important factor in this process is the fact that the footprint ofthe plasma discharge can be stable and reproducible, and dependant oncontrollable parameters. Fairly similar etch rates can be produced ifsimilar systems are run under identical conditions, and the same systemcan be highly reproducible from day to day. For extremely precisesurfaces, the footprint of the tool may need to be measured before eachremoval step. It may also be possible, however, to determine thefootprint as a byproduct of the iterative shaping process.

If any shape on the part is required, other than a Gaussian depressionof various depths, it may be necessary to translate and/or rotate thepart relative to the torch, although it may also be possible totranslate and/or rotate the torch with respect to the part, or both withrespect to each other. If the torch is held stationary and lowered intothe part a depression or pit may result. If the torch translates acrossthe part while spinning, a trench may be produced. The floor of thetrench can take on the characteristics of the distribution of reactivespecies in the torch, and also can be determined by how closely thetorch paths approach each other on subsequent passes. It may benecessary to move two stages at the same time. To accomplish this, asecond controller can be used, such as may be computer- ormachine-controlled. A basic system can be limited to a constant rotationspeed, with the translation speed across the part being controlled in astepwise fashion (i.e. go a certain distance at a fixed speed and at acertain point change the speed).

In such a process, a rough part can be measured for which a fairlyaccurate estimate of the footprint is known, such as from previousexperiments. The final desired part shape may be known, and a pathwayfor the tool can be calculated to get the final shape from all of theinput variables, including such input variables as initial part shape,plasma conditions, dwell time, and removal behavior of the workpiecematerial. When completed, the part shape could be accurately measuredand compared with the desired shape. The difference may be the error inthe assumption of the footprint shape.

To produce an approximation to complex (or flat) surfaces with such asystem, the part can be rotated as it is translated in front of thedischarge. For uniform material removal in certain applications, thespeed of the torch across the surface may need to be constant. For someapplications it may be necessary to vary all parameters simultaneouslyincluding tool position, part position, gas flow rate, gas flowcomposition and excitation energy.

Rapid Polishing of Rough Surfaces

One of the more surprising and interesting features of systems inaccordance with the present invention is the planarization and/orpolishing of rough surfaces. Parameters which can dictate the timerequired to polish glass or other suitable materials with the plasmasystem include the concentration of species in the plasma gas (bothreactants and products) and the temperature of the surface andsurrounding gas. Exchange of species on and off the surface, as well asthe local redeposition of material during etching, can be principallyresponsible for the rapid smoothing of rough surfaces, resulting inplanarization on at least a local scale.

The relatively high concentration of species in the plasma, and thelocal equilibrium established across the boundary layer by this process,can explain why other lower pressure plasma systems such as PACE do notexhibit such a smoothing effect. The higher pressure gas can reduce themean free path of the products, keeping the products in the surfaceregion for a greater amount of time. In addition, the higher pressuregas can have a greater heat capacity, keeping the near surface region ofthe solid at a higher temperature. While low pressure plasmatemperatures may be the same, the actual amount of heat deposited on thesurface using an atmospheric pressure plasma system can be greater dueto the higher flux of gas. This is evident in the fact that one systemin accordance with the present invention uses a 1.5 to 2.25 kW plasmawhile the PACE and microwave devices commonly run at a few hundred wattsin a maximum configuration.

Another way to change the amount of material available for deposition,and to affect the rate of planarization or smoothing, is to add areactant into the plasma that would cause deposition while the fluorineatoms cause etching. A combination of some volatile silicon compoundwith the addition of oxygen may be sufficient.

An equilibrium-deposition state in accordance with the present inventionis not the same as previous plasma deposition, as the process does notsimply fill in holes but rather involves a local redistribution ofmaterial at the surface. This may be important for applications where itis necessary that the structure of the final surface material be nearlyidentical to the bulk phase.

The foregoing description of preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Many modifications andvariations will be apparent to the practitioner skilled in the art. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical application, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with various modifications that are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalence.

1. A tool for processing a surface of a workpiece, comprising: a plasmaprocessing chamber in which the workpiece can be positioned; a plasmatorch included in the processing chamber, wherein the plasma torch isoperable to perform a plurality of operations to process the surface ofthe workpiece using reactive atom plasma processing; and a translatoroperable to translate at least one of the workpiece and the plasmatorch.
 2. The tool according to claim 1, wherein: the plasma torch is anICP torch.
 3. The tool according to claim 1, wherein: an operation inthe plurality of operations can be one of shaping, cleaning, polishing,and planarizing.
 4. The tool according to claim 1, wherein: the plasmatorch is operable to perform the plurality of operations while causingminimal or no damage to the workpiece underneath the surface.
 5. Thetool according to claim 1, wherein: the plasma torch is operable toproduce a volatile reaction on the surface of the workpiece.
 6. The toolaccording to claim 1, wherein: the plasma torch includes a multiple headto increase the etch rate of the plasma torch.
 7. The tool according toclaim 1, further comprising: a controller operable to perform at leastone of: injecting a precursor in the plasma torch in order to create areactive species from the precursor; controlling the etch rate of theplasma torch via the precursor; controlling the mass flow of theprecursor into the plasma torch; maintaining the temperature of theplasma torch between 5,000 and 15,000 degrees C.; maintaining theprocessing chamber at about atmospheric pressure, and using an auxiliarygas to adjust the position of a discharge.
 8. The tool according toclaim 7, wherein: the precursor can be any one of a solid, liquid, andgas.
 9. The tool according to claim 1, wherein: the plasma torchincludes an inner tube, an intermediate tube, and an outer tube.
 10. Thetool according to claim 9, further comprising: a controller operable toperform at least one of: introducing a precursor into the inner tube ofthe plasma torch; introducing an auxiliary gas into the intermediatetube of the plasma torch; introducing a plasma gas into the outer tubeof the plasma torch; coupling energy to a discharge in the inner tube ofthe plasma torch; and restricting the size of a discharge to the innerdiameter of the outer tube of the plasma torch.
 11. The tool accordingto claim 1, wherein: the translator includes: a translation stagecapable of rotating the workpiece with respect to the plasma torch; anda control system capable of being programmed to control the translationstage.
 12. A tool for shaping a surface of a workpiece, comprising: aplasma processing chamber in which the workpiece can be positioned; aplasma torch included in the processing chamber, wherein the plasmatorch is operable to establish an equilibrium in a plasma reaction inthe plasma processing chamber, whereby material may be removed from thesurface of the workpiece and re-deposited on the surface of theworkpiece with the discharge from the plasma torch; and a translatoroperable to translate at least one of the workpiece and the plasmatorch.
 13. A tool for cleaning a surface of a workpiece, comprising: aplasma processing chamber in which the workpiece can be positioned; aplasma torch included in the processing chamber, wherein the plasmatorch is operable to remove material from the surface of the workpieceusing reactive atom plasma processing; and a translator operable totranslate at least one of the workpiece and the plasma torch.
 14. A toolfor processing a surface of an optic, comprising: a plasma processingchamber in which the optic can be positioned; a plasma torch included inthe processing chamber, wherein the plasma torch is operable to performa plurality of operations to process the surface of the optic usingreactive atom plasma processing; and a translator operable to translateat least one of the optic and the plasma torch.
 15. The tool accordingto claim 14, wherein: the optic is a high-damage threshold optic.
 16. Atool for back-etching a wafer, comprising: a plasma processing chamberin which the wafer can be positioned; a plasma torch included in theprocessing chamber, wherein the plasma torch is operable to etch back asurface of the wafer with the discharge from the plasma torch usingreactive atom plasma processing; and a translator operable to translateat least one of the wafer and the plasma torch.
 17. A tool for thinninga wafer, comprising: a plasma processing chamber in which the wafer canbe positioned; a plasma torch included in the processing chamber,wherein the plasma torch is operable to thin the wafer by removingmaterial from a surface of the wafer with the discharge from the plasmatorch using reactive atom plasma processing; and a translator operableto translate at least one of the wafer and the plasma torch.
 18. A toolfor thinning a bonded wafer, comprising: a plasma processing chamber inwhich the bonded wafer can be positioned; a plasma torch included in theprocessing chamber, wherein the plasma torch is operable to thin thebonded wafer by removing material from an outer surface of the bondedwafer with the discharge from the plasma torch using reactive atomplasma processing; and a translator operable to translate at least oneof the bonded wafer and the plasma torch.
 19. A tool for shaping asurface of a workpiece at atmospheric pressure, comprising: a plasmaprocessing chamber in which the workpiece can be positioned; a plasmatorch included in the processing chamber, wherein the plasma torch isoperable to simultaneously remove material from the surface of theworkpiece and re-deposit the removed material back onto the surface ofthe workpiece at atmospheric pressure using reactive atom plasmaprocessing; and a translator operable to translate at least one of theworkpiece and the plasma torch.
 20. A tool for shaping a surface of aworkpiece, comprising: means for positioning a workpiece in a plasmaprocessing chamber including a plasma torch; means for translating atleast one of the workpiece and the plasma torch; and means forperforming a plurality of operations to process the surface of theworkpiece using reactive atom plasma processing.